1. Field of the Invention
The present invention relates to an information receiving system, adopting a diversity reception method and, particularly to a broadcast receiving system with a radio capable of receiving an AM broadcast and an FM multiplex receiver for receiving an FM multiplex broadcast, including a traffic information, according to a diversity reception method. The invention also relates to a method for controlling an information receiving system, in particular a broadcast receiving system.
2. Description of the Prior Art
Some of the recent car navigation systems are such that an FM multiplex broadcast, including traffic information, such as traffic regulations, a jam and a traffic accident, is received by an FM multiplex receiver and is displayed as character information or image information in a display device mounted near a driver's seat of a vehicle.
The FM broadcast radio wave received by the FM multiplex receiver reaches a receiving vehicle from a broadcast station through various paths, including being reflected by buildings, mountains and the like. Thus, a reception problem called "multipath distortion" occurs in the FM broadcast radio wave. A diversity reception method has been adopted to receive FM multiplex broadcast, and reduces the influence of multipath by switchingly selecting one antenna having a satisfactory receptive state from a plurality of antennas.
In the case that such a car navigation system is mounted on a vehicle, the FM multiplex receiver and a radio capable of receiving an AM broadcast are installed in proximity to each other. A noise generated by switching the antennas of the FM multiplex receiver leaks from the antennas of the FM multiplex receiver and is imported onto a signal received by the radio, with the result that the noise is imposed on the voice outputted from the radio. Further, as generally known, the AM broadcast is an amplitude modulation and is thus more easily affected by the noise which acts on the amplitude than the FM broadcast. Accordingly, the influence of the noise generated by switching the antennas is larger when receiving the AM broadcast than when receiving the FM broadcast.
A prior art for reducing the influence of the switching of antennas on the radio is such that a switch pulse for instructing the switching of the antenna is inputted to a diode which acts to switch the antennas via an integral circuit comprised of a capacitor and a resistor so as to reduce a noise by smoothing the rise and fall of the switch pulse (Japanese Unexamined Patent Publication No. 56(SHO)-168440).
With the prior art disclosed in the above publication, the noise generated during the switching of the antennas may be reduced considerably, but cannot be completely eliminated. Accordingly, in the case that a radio is installed in a very short distance from the receiver of this prior art, e.g. in the same vehicle, if the antennas are switched while the radio is receiving an AM broadcast, an AM tuner is easily affected by the noise as mentioned above. Thus, the AM tuner is affected even by a reduced level of noise, with the result that the noise is inadvertently imposed on the outputted voice.
An object of the present invention is to provide an information receiving system and a control method therefor which are capable of preventing the influence of a switch noise generated during the switching of antennas by a receiver provided with a plurality of switchable antennas from deteriorating a receptive state of an other receiver.